In a toroidal type continuously variable transmission for a vehicle as disclosed for example in U.S. Pat. No. 5,542,890, a speed change ratio varies according to a gyration angle of a plurality of power rollers gripped between an input disc and an output disc. These power rollers are each supported by a trunnion.
The gyration angle of the power roller varies when a force in a specified direction at right angles to both the power roller rotation axis and the disc rotation axis, acts upon the trunnion.
This variation depends on a load applied by the input disc and output disc to the power roller due to a displacement of the power roller in the aforementioned specified direction. The dynamics of this gyration angle variation is explained for example in SAE Technical Paper NO. 901761.
In such a transmission, when an external force in the specified direction acts on one of the power rollers due to an unexpected disturbance, the gyration angle of the power roller varies as a result.
When the gyration angle changes, the transmission torque of this power roller varies, and the force exerted by the input and output discs on the power roller also varies. In this way, torque is transmitted from the input disc to the output disc at a new balance point when the force due to the external disturbance, the force applied by the input and output discs to the power roller depending on the transmitted torque and the oil pressure supporting the power roller via the trunnion, are in equilibrium.
In a power roller control mechanism, e.g. as disclosed in Tokkai Hei 3-89066 published by the Japanese Patent Office in 1991, a plurality of power rollers are supported by an identical oil pressure controlled by a control valve. A gyration angle and offset amount of a specific power roller is fed back to the control valve via a precess cam which responds to both of these parameters.
The reason why the offset amount of the rotation axis is fed back in addition to the gyration angle in this way, is because if only the gyration angle were fed back, hunting of the controlled oil pressure would easily occur.
The purpose of this feedback control is to maintain the gyration angles of all the power rollers identical. However when the above specified power roller is displaced in the offset direction due to an external disturbance, it also affects the oil pressure supporting other power rollers not subject to the effect of the disturbance, because oil pressure control of all the power rollers is performed based on the offset amount of the specified power roller.
As a result, synchronous operation of the power rollers may collapse, and this may lead to vibration of the power rollers.